This invention relates to a nuclear magnetic resonance diagnostic apparatus which utilizes the nuclear magnetic resonance phenomenon (referred to as "NMR" phenomenon hereinafter) so as to noninvasively measure information concerning the spin density and relaxation time of a specific atomic nucleus or a proton within a selected sectional slice plane of an object to be examined, e.g., a patient, for which a tomographic image is obtained with a high S/N ratio.
First, the principle of an NMR apparatus will be summarized.
Atomic nuclei are composed of protons and neutrons. It is generally considered that they are spinning as a whole like a top. In other words, an atomic nucleus of hydrogen (.sup.1 H) is comprised of one proton which is spinning in a manner indicated by spin quantum number 1/2 as shown in FIG. 1A. Also shown in FIG. 1B, since the proton holds a positive charge (e.sup.+), rotating nuclei of hydrogen may be considered equivalent to a current corresponding to the above positive charge flowing in a small coil. As a result, a magnetic moment .mu. occurs. In other words, a respective nucleus of hydrogen can be regarded as a very small magnet. In general, as schematically shown in FIG. 2A in ferromagnetic materials such as an iron, all of the very small magnets are oriented in the same direction, so that a macroscopic magnetization "37 M" can be observed. To the contrary, since each of the magnetic moments in the nucleus of hydrogen is oriented at random, the macroscopic magnetization cannot be observed as shown in FIG. 2B. If a static magnetic fields H.sub.O is applied to the nuclei, each of the nuclei is directed toward a magnetization direction of the field H.sub.O (,i.e., an energy level of the nucleus is quantized in the Z direction). This condition of the nuclei of hydrogen is displayed in FIG. 3A. As the nucleus of hydrogen has 1/2 quantum number, the nuclei of hydrogen are divided into two energy levels, i.e., -1/2 and +1/2. Most of the divided hydrogen nuclei are oriented in the Z direction corresponding to +1/2 energy level.
A difference between these two energy levels is given by formular (1); EQU .DELTA.E =.gamma.H.sub.O ( 1)
where .gamma. is a gyromagnetic ratio (the ratio between the magnet and mechanical moments), h is a Plank's constant and is equal to h/2.pi..
Since the static magnetic field H.sub.O is being applied to each of the hydrogen nuclei so that a force indicated by .mu..times.H.sub.O is applied thereto, each of the hydrogen nuclei rotates around the Z axis at an angular velocity of .omega.=.gamma.H.sub.O (i.e. the Larmor angular velocity). Under these conditions of the nuclei of hydrogen when an electro-magnetic wave (normally a radiofrequency wave) having a frequency corresponding to the angular velocity .omega. is applied, a nuclear magnetic resonance occurs. As a result, the nuclei of hydrogen absorb an energy .gamma..multidot.H.sub.O which corresponds to the above-mentioned energy level difference (.DELTA.E), so that transition of the nuclei of hydrogen occurs to a higher energy level. Although there exist several kinds of nuclei in one object which have their own respective spin angular momentum, it is possible to pick up a resonance of a specific atomic nucleus only, because each of the nuclei has its specific gyromagnetic ratio .gamma., and each of them has different resonance frequency. Moreover if an amplitude of the resonant signal is measured, the density of the atomic nucleus in the object can be obtained. The nucleus which has been excited to the high energy level returns to the lower energy level after the occurence of the nuclear magnetic resonance in a period of time that is defined by a time constant (i.e. the so-called "relaxation time"). The relaxation time includes a spin-lattice relaxation time "T1" and a spin-spin relaxation time "T2". The spin-lattice relaxation time "T1" and the spin-spin relaxation time "T2' are such time constants that they are decided depending upon the combination of the composition of the object. For example, values of those relaxation times for the normal tissue are different from that for the malignant tumor.
Although the above description will cover only hydrogen-1, it is obvious that similar measurements can be applied to other atomic nuclei having spin angular momentums different from that of hydrogen-1. For example in the normal chemical analysis, nuclei of flourine-19, of phosphorus-31 and carbon-13 are utilized.
As described hereinbefore in detail, since the density and relaxation times of the specific atomic nucleus are measured by utilizing the NMR phenomenon, chemical information of this nucleus can be obtained.
It should be noted that the NMR signals introduced in the present specification involve echo pulses, or echo pulse signals and also free induction decay signals (referred to as "FID signals" hereinafter). The following embodiments will involve only the echo pulse signals.
There is known "a spin echo method" as one of measuring methods for utilizing these echo signals. According to this spin echo method, an "echo" signal of the NMR signal is measured after 2.tau. time periods by using 90.degree.-.tau.-180.degree. -2.pi.-180.degree.-2.pi.-180.degree. pulse series, ".tau." being a predetermined wait time. It is understood that angles of 90.degree. and 180.degree. of the applied pulses are determined by the following equation (2) under the strength of the applied magnetic field and the applied time of the pulse "tp"; EQU .theta.=.gamma.H.sub.1 tp[rad] (2)
As is well known, there is a Nuclear Magnetic Resonance-Computerized Tomographic Apparatus (referred to as "NMR-CT apparatus") in which using this echo signal, a distribution of the spin density of a specific atomic nucleus in a certain imaginary slice of the object is processed in the a computer so as to reconstruct a tomographic image of the slice. According to a recent development in this technical field, a phase detection technique is newly introduced in order to utilize frequency information of the detected echo signals. However there are still difficulties that the echo signals are very weak, and random noises caused by the receiver channels and the object are superimposed to the above-described very weak echo signals, resulting in a low signal-to noise (S/N) ratio. Consequently there exists an extreme difficulty in that only pure signals induced by the NMR phenomenon are selectively detected.
It is therefore an object of the present invention to provide an NMR diagnostic apparatus in which an S/N ratio of the NMR signal induced by the nuclear magnetic resonance phenomenon can be improved.